The usage of modern plastics and air-recycling systems in the buildings require eliminating of the associated side-effects. Namely it is the slow release of undesirable aldehydes and roughly two hundred of other organic substances; and infections spreading through the HVAC systems into the whole building.
The fundamental functionalities of traditional paints and pigments always lie in the color and hiding power. However, there are an increasing number of attempts to add other functionalities to paints. From the historical point of view, the first such paint was lime, which, in the fresh form, functions as a white pigment and as a sanitary paint as well. Applications of the fundamental photocatalytic effect of TiO2 nanoparticles are very popular in the last decade. Although the effectiveness of the photocatalytic effect of TiO2 was examined very well, all problems with the application of TiO2 in photoactive paints have not been satisfyingly solved yet.
If TiO2 nanoparticles are mixed into the inorganic silicate paints, the surface of nanoparticles is typically encapsulated by silicon oxide (SiO2) and the desiderative photocatalysis is blocked. This surface modification of TiO2 pigment particles by SiO2 is used in the pigment industry for almost a century to decrease the photocatalytic effect preventing the paint from chalking. The photocatalytic effect of the silicate based products is in the best case residual and it amounts to only percents or tenths of percent of the pure TiO2 surface photocatalytic activity. TiO2 nanoparticles are usually used to improve the rheology of the paint rather than for its photocatalytic effect in these types of composition.
The second most frequent practice of incorporating TiO2 nanoparticles into the paint composition is to mix them directly into the acrylate based paints. The problem of such solution is in the photocatalytic aggresivity of TiO2 nanoparticles, which upon illumination photochemically break down and literally burn the surrounding acrylate. The result is strong chalking and yellowing of the paint.
Utilization of TiO2 nanoparticles in the silicone based paints also brings serious limitations. Silicones, similar to silicates, effectively block the TiO2 surface eliminating the photocatalysis.
The application of sol-gel made TiO2 nanoparticles directly on a surface is very costly and rather uncommon. The thickness of sol-gel TiO2 spray coating is around 50 nm, Nanoparticles of TiO2 stick to the wall electrostatically. The limitations of this technology are in the low purity of sol-gel TiO2 nanocrystals and high acidity of the typical sol-gel precursors. Another problem is the low TiO2 quantity in the thin coating that uses only a minimal fraction of the available light.
In the so far known paints based on the photocatalytic effect of TiO2 nanoparticles, the photocatalysis is diminished because nanoparticles get surrounded by a substance contained in the paint composition, typically the binder. This slows the photocatalytic degradation of organic substances, especially cigarette smoke, some thinners, exhalations and aldehydes vaporized from the plastics on the painted surface.